In recent year, with the flourishing development of semiconductor technology, portable electronic products as well as flat panel display products have also been raised. A flat panel display usually is consisted of pixel matrixes arranged in both vertical direction and horizontal direction. When performing display function, the flat panel display generates gate input signals through a shift register and scans each row of pixels in an order from a first row to a last row. In designing the flat panel display, it needs to design an appropriate shift register to ensure stable operation thereof. Usually, the shift register is comprised of multiple stages of shift register units which are connected in series, and an output signal of a previous stage of shift register unit is used as an input signal of a subsequent stage of shift register unit.